Many signal processing and radio telecommunications systems make use of both radio frequencies (RF) and intermediate frequencies (IF) during operation. For example, AM and FM radio use an RF data signal with an IF carrier signal. In addition, some radar or microwave telecommunications applications also make use of RF and IF signals. In such devices a higher-frequency RF signal is transmitted over the air, while a lower-frequency IF signal is used for modulation/demodulation within transmitter and receiver circuitry.
Conversion between RF and typically takes place in a frequency converter that receives as its inputs a local oscillator (LO) signal and a signal for conversion (i.e., one of the IF signal and the RF signal), and provides as an output the frequency-converted signal (i.e., the other of the IF signal and the RF signal). Such a frequency converter may also be called a mixer.
In operation, if a frequency converter needs to upconvert an IF signal to an RF signal, it will receive the LO signal and the IF signal as inputs, and will provide the RF signal as an output. Likewise, if the frequency converter needs to downconvert an RF signal to an IF signal, it will receive the LO signal and the RF signal as inputs, and will provide the IF signal as an output. One exemplary frequency converter is the star mixer in Schottky diodes arranged in a star configuration.
Many modern communication systems require a high dynamic range of frequency converter operation, however. This means that a frequency converter for such devices must minimize distortion of IF and RF signals.
One limitation for meeting this high dynamic range requirement is the linearity of conventional frequency converters, i.e., the range for which the conventional frequency converters operate in a linear manner. Many conventional frequency converters have poor linearity performance in that they cannot provide the dynamic ranges required by some systems. For instance, a star mixer has a third order intermodulation that is about the same as a given LO drive level. This results in intermodulation distortion (IMD) and spurious responses, indicating that the star mixer has a relatively small range of linear operation.
It would therefore be desirable to provide a high linearity mixing circuit that provides linear operation over a wide range of RF and IF signal levels.